Method of making powdered compacts



United States Patent 2,768,099 METHOD OF MAKING POWDERED COMPACTS NormanS. Hoyer, Pittsburgh, Pa., assignor to Gibson Electric Company,Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. ApplicationOctober 16, 1952, Serial No. 315,210

6 Claims. (Cl. 117-213) My invention relates to circuit breakercontacts, and more particularly to a novel method of manufacturingcircuit breaker contacts to prevent the bleeding out of the infiltrantduring the impregnation step commonly employed in the manufacture ofpowdered metal material.

Electrical contacts of the type to which my invention relates arecomposed of composition impregnated with a conducting metal, suchcompositions including powdered refractory material plus a highlyconductive metal of lower melting point such as silver or copper. Therefractory material when used may be tungsten, molybdenum and/or their(or other) carbides. This powder of refractory material with or withoutsome lower melting point metal such as silver or copper is placed in amold having the desired shape of the contact of the end result. Thematerial is then pressed into the form of a compact as is now well knownin the art. This pressed material is then sintered at temperatures andfor periods sufficiently large and long to permit the particles of thepowder to fuse or sinter together. This step in the manufacturing ofcompacts from a powdered refractory is old and well known in the art.

After the above steps have been completed, it is necessary to impregnateor infiltrate the resulting porous compact with an infiltrant of ahighly conductive metal, such as copper or silver. This treatment of theinitial porous sintered combination comprises the operation ofimpregnating the same with a highly conductive metal. One such method,well known in the art, is to place a slug of silver on the porouscomposition and heating this combination until the silver has melted. Atthis point, the highly conductive silver metal will infiltrate into thepores of the porous sintered composition.

Following this step, the contact may be prepared and welded to a contactarm of the circuit breaker in any desirable manner and forms no part ofmy invention. This method of making an electrical contact of refractorymetal powder such as tungsten, molybdemun, etc. or their carbides, withor without some lower melting point metal, to form a porous compact,sintering the compact and impregnating this compact with silver orcopper, is old and well known in the art.

The refractory compact can absorb its infiltrant of silver or copper byany one of several methods. One such method is immersing the porousrefractory compact into a molten bath of silver or copper and allowingthis molten metal to flow into the porous refractory compact. Anothermethod of infiltration, well known in the art, is to place theinfiltrant in a solid form on the compact or to place the compact on theinfiltrant, and expose the combination to a suflicient degree of heat tomelt the infiltrant and thereby enable it to seek its way into the poresof the refractory compact.

Whichever method is used, the silver or copper permeates into thecompact, generally in a very short time such as a matter of minutes.However, the length of time required for the infiltrant to permeatethroughout the entire compact is determined by the size and thickness ofthe compact as well as by the area and volume of the porous openings.

In practice, it has been found that where the infiltration time wasexcessive, the infiltrant would begin to leak or ooze out of therefractory compact and then attach itself to the sides and the bottom ofthe compact in the form of mounds or globules. This, obviously, is avery undesirable occurrence since increased bleeding of the infiltrantincreases the porosity therein.

Furthermore, the weight of infiltrant is usually precalculated so thatgenerally the amount of the infiltrant that is used is slightly greaterthan sufiicient to fill all of the porous channels of the refractorycompact. Hence, the amount of the infiltrant which oozes out of thecompact is a portion of the amount which was calculated to fill theporous openings thereof and thus a non-uniform contact will result.

The problem of bleeding during infiltration of powdered metal materialshas been recognized for many years in the industry and many methods havebeen proposed to overcome it. One such method is to calculate the exacttime required for the infiltration step and thus stop the process ofinfiltration before bleeding commences. However, this has been found, asabove noted, to be very undesirable due to the inaccuracies anddificulties encountered in computing the exact time required.Furthermore, it has not been found an efficient or desirablemanufacturing expedient to place exact time limits on the infiltrationstep.

The undesirable resulting effect is then eliminated by grinding, sandingor machining the product to remove the resultant mounds or globulescaused by the bleeding. Although the undesirable globules are removed inthis manner, it obviously is not a desirable step as it is a costlytime-consuming procedure, and furthermore, requires additional steps inthe manufacturing process.

I have found by experimentation that if the exterior portion of theporous refractory compact has graphite or lampblack applied thereto, abarrier will be erected to prevent the conductive material from oozingor seeping out of the pores of the compact.

With the method of my invention, it will be possible to infiltrate apowdered refractory compact with a predetermined weight of conductivematerial and the length of time of the impregnation step need not becritical.

Accordingly, my invention is to apply a coating of carbon to allsurfaces of a compact, with the exception of the surface on which theinfiltrant is placed and thereby prevent the bleeding out of theinfiltrant during impregnation.

Accordingly an object of my invention is to produce an infiltratedsintered material on which there are no globules or mounds of conductingmaterial due to bleeding during the infiltration step.

Another object of my invention is to provide an electrical contactwhich, when manufactured, never has excess amounts of conductingmaterial attached to its outer surface.

A still further object of my invention is to provide a novel method ofinfiltration of porous refractory material such that the critical timeperiod for infiltration is removed.

A still further object of my invention is to provide means whereby it isunnecessary to time accurately the infiltration period in order to avoidundesirable oozing out of the conductive material.

A still further object of my invention is to provide a novel method ofimpregnating porous compacts whereby a coating of carbon is applied toall of the area, with the exception of the area covered by the solidconductive material, prior to the impregnation step.

These and other objects of my invention will appear from the followingdetailed description.

In the field of powdered metallurgy, many electrical contacts are formedfrom a refractory in powdered form. This powder is compacted into a dieand the appropriate pressure is applied to obtain a precalculateddensity. The compact is generally sintered and then exposed to aninfiltration step wherein a metal of lower melting point than therefractory, known as the infiltrant, is absorbed into the pores of thispowdered metal compact. The infiltration temperature is at or above themelting point of the infiltrant.

In electrical contacts, an infiltrant of silver or copper is absorbedinto the porous refractory compact resulting in an electrical contactmaterial which is generally used for large current applications.

One common method of infiltration is to place the infiltrant in solidform on the compact or place the compact on the infiltrant and subjectthis combination to a temperature which is sufficieut to melt theinfiltrant. This permits the silver or copper to permeate throughout thecompact and fill the pores thereof. After filling these pores, continuedinfiltration time only results in the bleeding out of some of theinfiltrant.

My invention is directed to a method or process for preventing thebleeding in such infiltrated electrical type materials.

I have found that the rubbing of carbon, generally in the form ofgraphite or lampblack on all exposed surfaces prior to impregnation willprevent the infiltrant from oozing or bleeding out of the pores of thecompact; that is, a coating of carbon may be applied to all surfaces,with the exception of the surface on which the infiltrant is placed,which will thereby act as an obstacle or barrier for the impregnatingmaterial and thus prevent it from oozing to the exposed surfaces andforming mounds or globules thereon.

Thus, it becomes unnecessary (l) to control accurately the time periodof impregnation before bleeding will occur; and (2) to eliminate thenecessity of removing the excess globules which previously were formedwhen bleeding occurred.

Thus, by providing the compact with a coating of carbon, I can removethe critical time period of impregnation and still allow for ampleinfiltration time without worry of bleeding.

It will be noted that the carbon coating of graphite or lampblack may beapplied to the exposed surface of the compact by rubbing it on with thehand. However, I have also found that it is desirable to apply thecoating by preparing a mixture of the graphite or lampblack in carbontetrachloride or some other similar vehicle.

Thus, this mixture may then be painted on to the exposed surfaces of therefractory compact. This method of application is very desirable forcompacts having hard to reach areas such as radii undercuts orstep-downs. Furthermore, the carbon tetrachloride which evaporatesreadily, has no deleterious effects during infiltration or on theproperties of material. It has also been found that a more desirable andbetter coating or smearing efiect on the exposed surface results whenfiner particles of carbon are used.

In summary, I provide a novel impregnation method in which the exposedsurfaces of a sintered refractory compact are coated with a carbonsurface and thereby eliminate all deleterious bleeding effects, alsoeliminating the critical time of infiltration.

Although for purposes of illustration I have described my invention asapplied to a refractory composition impregnated with a lower meltingpoint conducting metal, it will now be evident that my invention isapplicable generally to materials in which impregnation is employed andwhere it is desired to prevent bleeding of the infiltrant.

I claim:

1. In a process wherein a porous article made by sintering a compact ofpowdered refractory material is, after sintering, impregnated with amolten metal material having a melting point lower than that of therefractory material, the improvement which comprises applying to aportion only of the surface of the sintered compact before impregnationa coating of finely divided carbon selected from the group consisting ofcarbon black and graphite.

2. The process of claim 1 in which the compact is coated with lampblack.

3. The process of claim 1 in which the compact is coated with graphite.

4. In a process wherein a porous article made by sintering a compact ofpowdered refractory material from the group consisting of tungsten,molybdenum, carbides of tungsten and molybdenum, and mixtures thereofis, after sintering, impregnated with a molten metal from the groupconsisting of silver and copper, the improvement which comprisesapplying to a portion only of the surface of the sintered compact beforeimpregnation a coating of finely divided carbon selected from the groupconsisting of carbon black and graphite.

5. The method of claim 4 in which the finely divided carbon coating isapplied by suspending the same in carbon tetrachloride and subsequentlyevaporating the carbon tetrachloride.

6. In a process wherein a porous article made by sintering a compact ofpowdered refractory material from the group consisting of tungsten,molybdenum, carbides of tungsten and molybdenum, and mixtures thereofis, after sintering, impregnated with a molten metal from the groupconsisting of silver and copper, the improvement which comprisesapplying to all exposed surfaces of the compact, with the exception ofthe impregnating surface, a coating of finely divided carbon from thegroup consisting of carbon black and graphite.

References Cited in the file of this patent UNITED STATES PATENTS2,180,984 Hensel Nov. 21, 1939 2,427,517 Wilson Sept. 16, 1947 2,466,927Burton Apr. 17, 1949 2,606,831 Koehring Aug. 12, 1952 FOREIGN PATENTS650,555 Great Britain of 1951

1. IN A PROCESS WHEREIN A POROUS ARTICLE MADE BY SINTERING A COMPACT OFPOWDERED REFRACTORY MATERIAL IS, AFTER SINTERING, IMPREGNATED WITH AMOLTEN METAL MATERIAL HAVING A MELTING POINT LOWER THAN THAT OF THEREFRACTORY MATERIAL, THE IMPROVEMENT WHICH COMPRISES APPLYING TO APORTION ONLY OF THE SURFACE OF THE SINTERED COMPACT BEFORE IMPREGNATIONA COATING OF FINELY DIVIDED CARBON SELECTED FROM THE GROUP CONSISTING OFCARBON BLACK AND GRAPHITE.